1. Field of the Invention
The present invention relates to a method and an apparatus of driving an inkjet printer, and more particularly, to a method and an apparatus of flexibly controlling print quality and throughput of an inkjet printer.
2. Description of the Related Art
A thermal bubble inkjet printer is one of the most popular printer types for the time being. The method of controlling inkjet printhead has been disclosed by U.S. Pat. No. 5,604,519, owned by Hewlett-Packard Company, titled “Inkjet Printhead Architecture for High Frequency Operation”. FIG. 1 is a drawing showing an internal circuit of a prior art nozzle at an inkjet printhead of an inkjet printer. The nozzle of the inkjet printhead in FIG. 1 comprises a heater resistor, an address signal line 102 and a primitive signal line 104, wherein the address signal line 102 and the primitive signal line 104 are used to drive the nozzle. The heater resistor is connected to high voltage only when both the address signal line 102 and the primitive signal line 104 are driven, which in turn, generates heat to boil the ink. While the heater resistor 100 is heated to vaporize and eject the ink stored in the ink chamber of the nozzle, simultaneously driving the address signal 102 and the primitive signal line 104 is required. On the nozzle surface of the inkjet printhead, nozzles are divided into several nozzle primitive areas. A group of neighboring nozzles constitute a nozzle primitive. Each nozzle of the nozzle primitive has its own address. According to the selected address signal line 102 and the nozzle primitive signal line 104, the selected nozzle is driven for inkjet. In every nozzle primitive, only one nozzle is driven. It means that each address signal line 102 is independently and sequentially driven until all of the address signal lines 102 are driven. All of the nozzle positions of the inkjet printhead are scanned once. The primitive data then are transmitted to the nozzle primitive signal lines 104 so that the selected nozzle correctly ink jets. FIG. 2 is a drawing showing a relationship between address signals and nozzle primitive signals. When an address signal line is driven, the corresponding primitive data are transmitted to the nozzle primitive signal line. FIG. 3 is a drawing showing a driving of an inkjet printhead with seven address signal lines A1-A7. Referring to FIG. 3, all of the address signal lines are driven sequentially. The description above is the operation of the basic driving for the thermal bubble inkjet printhead.
During an inkjet print swath, a print controller controls the above operation during the period between each column. The horizontal resolution of columns depends on the desired horizontal print resolution (pixel resolution, for example, 600 dpi). In other words, the print controller drives each address signal line, loads the primitive data to the corresponding nozzle primitive signal line, and drives the nozzle to jet ink dots through a pen driver. Determining whether the printhead reaches the pixel column is fed back from an encoder strip sensor of a carriage when the printhead passes through the print swath. The feedback is the encoded signal Enc of the pixel area in FIG. 3. There are seven address signals A1-A7, which are to be sequentially driven between two pixels. The waveform width of the nozzle primitive signal in the address signal waveform must be so large that a desired energy is provided to the nozzle heat resistor to form and jet the optimized droplet shape and size. Accordingly, the address signal waveform width corresponding to the design of the printhead is almost a constant. This constant but might be different for different print heads, for example, it needs from 2 us to 4 us to drive HP heads of different generations. When the print resolution is increased, or the moving speed of the inkjet printhead is accelerated, the time for the inkjet printhead to pass through two pixels is reduced. The time is also limited by performing the sequence of the address signals so as restrain the print throughput.
For a general inkjet printing, print quality and print throughput affect each other. For a printing with micro ink droplets, such as smaller than 5 pico liter, to diversify the color level changes, a multi-pass print method is required. According to multiple layers of ink jetting colors of the multi-pass print method, the desired print quality of pictures can be obtained. This method, however, reduces the print throughput. Regarding the multi-pass printing, references can be obtained from U.S. Pat. No. 4,963,882 and U.S. Pat. No. 5,469,198, owned by Hewlett-Packard Company.
Among the variety of print modes, the requirement for the print throughput of the draft print mode is very high. Once the draft print mode is selected, the user expects seeing the result of the print object in a very short period of time. Similarly, the expectation of the print quality of the object is relatively reduced. According to the U.S. patent applications, it is found that the worldwide leading printer companies have committed to enhancing and improving the print throughput of the draft print mode.
FIG. 4A is a drawing showing a print encoded signal Enc and an address signal waveform according to a prior art draft mode. During the printing swath, if the moving speed of the inkjet printhead is increased, the frequency of the encoded feedback signal of the pixel position will also be increased. It means that the time between two neighboring encoded signals is reduced. As described above, to maintain the optimized ink droplet shape and size, the waveform width of the address signal and the inkjet primitive signal should be maintained at a constant. Referring to FIG. 4A, the time 40 at driving the signal A7 has passed the encoded signal feedback period that causes the encoded feedback signal 41 of the pixel being omitted. The print controller does not access and print the data stored in the encoded feedback signal of the omitted ones until the next encoded signal comes. Accordingly, the print horizontal resolution becomes half of what it should be. This data print error results in the horizontal extension of the print output image. For example, the print throughput under the draft print mode is restrained. The restrained moving speed of the inkjet printhead causes the print output image in error. A value over the restrained speed would cause the error printout. Of course, the distorted output image is unacceptable by users.
In order to solve the issue mentioned above, U.S. Pat. No. 6,315,388, owned by Hewlett-Packer Company, titled “Draft Printing” discloses a solution method. According to this method, half of the address signals are omitted in each pixel column. Referring to FIG. 4B, only the address signals A1, A3, A5 and A7 are used in the first pixel column, and the address signals A2, A4, A6 and A8 are used in the next pixel column. Accordingly, the print throughput of the inkjet printhead is doubled and the purpose of enhancing the print throughput is achieved. Under this patent, the the half of nozzles would be disabled (since their address signals are ignored). For one hundred percent coverage data, the ink dots must be exactly reduced by half. The enhanced print throughput is fixed. A user can not select a balance between the print quality and print throughput. Additionally, the moving speed of the inkjet head is still limited by the sequence of the halved address signals.